EARLY DOLOMITIZATION, ANCIENT BRINE SEEPS, AND THE BURGESS SHALE AT MONARCH MOUNTAIN, SOUTHEASTERN BRITISH COLUMBIA

The Monarch, a mountain in SE British Columbia, exposes the Cathedral Escarpment which abuts the basinal Burgess Shale and an underlying limestone. At one section, a megabreccia forms the Escarpment wall, and includes dolostone clasts up to 90 m in diameter. Some workers have interpreted these as olistoliths spalled from the Escarpment into the adjacent basin. However, the breccia is more likely an intraplatformal cavern collapse feature associated with early dolomitization because: 1) the clasts are surrounded with saddle dolomite and sulfide minerals, not basinal sediments as expected if the clasts were exposed on the seafloor following margin collapse; 2) the megabreccia includes varied dolostone clasts (some fabric-preserving, some not) but lacks clasts of abutting basinal rocks and so cannot be interpreted as a post-Middle Cambrian subsurface diagenetic feature; 3) despite the size of the contained clasts, the megabreccia shows a restricted lateral distribution, not the apron-like geometry of a debris flow deposit; and 4) the megabreccia is directly overlain by non-dolomitized basinal limestone. The simple cross-cutting relationship of the limestone shows that brecciation and the surrounding saddle dolomite cementation must have occurred prior to margin collapse. If so, magnesium rich-brines were flowing within the Cathedral Margin, probably along deep-seated faults, during the Middle Cambrian. Alleged olistoliths in the Burgess Shale at this locality are, in fact, basin-derived carbonate mudmounds.

Further evidence for early brines at this locality includes lenses of magnesium-rich chlorite (> 30wt% MgO vs ~ 2% in background shales) that occur within the Burgess Shale next to the Escarpment wall. These are best interpreted as seafloor brine deltas and brine pools analogous to ones currently found at the foot of the Florida Escarpment and in the Gulf of Mexico, respectively. Like the modern examples, the Burgess Shale brine pools hosted abundant organisms in a narrow zone around their peripheries but not within the pools. Sponges, priapulid worms, and hyoliths predominate. Shales only a few metres basinward from the pools are nearly barren of animal fossils. Therefore, fluid exhalation was a major factor controlling fossil distribution, and at least some Burgess communities were supported by chemosynthesis.